Main signal and presignal
The main signal and the appropriate presignal characterize the setting of railway. But both of them do not have good prospects, because more and more cab signals substitute the trackside signals, so that these get obsolete. Nevertheless, the appreciation for the alternative signal system for railway is relevant for future. Main signals reveal state of the following block.
In other words, they predicate if the referred block is occupied, reserved or in its basic state. The presignal is an additional support in case of middle and high trackside velocities or in case of restricted of view distance. They enable a timely information about the state of the referred block by reproducing the same content as the main signal.